In the radio frequency field based electron, proton and ionized particle accelerators involving continuous wave or long-pulse duration beams, and accelerating fields (or gradient) above a few million volts per meter (MV m−1), SCRF cavities bring various advantages over the conventional copper cavities. As a result, in recent years, SCRF accelerating structures are being deployed in various high energy accelerators all over the world.
Most SCRF cavities currently used are based on Niobium (Nb). The quality of SCRF cavities depend to a very great extent on the material characteristics.
Cold test studies of certain Niobium SCRF cavities have shown that these cavities sustained an electric field gradient in the range of 26-35 MV/m. Such range of electric gradient corresponds to magnetic field (experienced by the SC-RF cavity surface) in the range of 1.0 to 1.6 kOe.
U.S. Patent Application 20060219336 discloses Niobium cavities, which are fabricated by the drawing and ironing of as cast niobium ingot slices rather than from cold rolled niobium sheet. This method results in the production of niobium cavities having a minimum of grain boundaries at a significantly reduced cost as compared to the production of such structures from cold rolled sheet.
U.S. Pat. No. 4,857,360 teaches a process for the manufacture of superconducting cavity resonators with improved surface quality, whereby even complex shaped cavity resonators can be made with cavities coated with NbN.
U.S. Pat. No. 7,151,347 discloses a niobium cavity exhibiting high quality factors at high gradients. This is achieved by treating a niobium cavity through a process comprising: 1) removing surface oxides by plasma etching or a similar process; 2) removing hydrogen or other gases absorbed in the bulk niobium by high temperature treatment of the cavity under ultra high vacuum to achieve hydrogen outgassing; and 3) assuring the long term chemical stability of the niobium cavity by applying a passivating layer of a superconducting material having a superconducting transition temperature higher than niobium thereby reducing losses from electron scattering in the near surface region of the interior of the niobium cavity. According to a preferred embodiment, the passivited layer comprises niobium nitride (NbN) applied by reactive sputtering.
While the above state of the art reveal the importance attached to the fabrication of SCRF-cavities for high accelerating gradient, importantly, the qualification process of such SCRF cavity material is now solely based on the enhancing of the residual resistivity ratio (RRR) of the Niobium through various kinds of material treatments.
Importantly, with high residual resistivity ratio (RRR>300) Niobium as the starting material, the existing techniques of SCRF “cavity fabrication” and “cavity surface treatment” have successfully taken care of extrinsic effect like electron-loading, multipacting, Q-disease etc. However, it remains a matter of fact that all the SCRF cavities fabricated with the same process do not always deliver high accelerating fields. This leads to a lot of wasted effort and materials. Therefore, there is a continuing need to develop suitable material qualification scheme, which would ensure that most, if not all, of the fabricated cavities deliver high accelerating fields.
Moreover, there is no method of SCRF cavity material qualification involving the determination of the magnetic field limit of the Niobium SCRF cavities prior to cavity fabrication procedure. Thus the presently known processes of qualifying such super conducting materials through residual resistivity ratio are not adapted to facilitate reliable achievement of the highest possible peak RF-magnetic field Hpeak in the SCRF accelerating cavities.
Due to the above limitations of the conventional method of qualifying Niobium materials the same also lead to wastage of the Niobium materials because there is no possibility of choosing the right and properly qualified Niobium materials which would favour the desired reproducible production of the SC-RF cavities and thereby improve the efficacy and yield of such cavity fabrication process.
Also, the present methods for qualifying Niobium and other materials for the production of SCRF-cavities do not take account of any possible influence of the thermal, mechanical and chemical treatment on the magnetic field limit of SCRF cavity material.